DIGESTION IN STOMACH 



107 



Inner surface of stomach 



Parietal cell 

 & (acid 



secreting) 



Central cells 

 (pepsin 

 secreting) 



a ferment 1 called pepsin. During the digestion of a 



meal, a large amount of gastric juice is secreted and is 



thoroughly mixed with the food by the contractions of 



the stomach, forming a grayish acid liquid called chyme. 



The gastric juice changes the pro- 



teids in the food into soluble forms 



which can be readily absorbed, 



called peptones, but it has no 



effect upon the carbohydrates or 



hydrocarbons. 



Exit of chyme from the stomach. 

 During the first hour or .so of 

 digestion, the opening from the 

 stomach, the pylorus, is compara- 

 tively small. Only the liquefied 

 contents, such as the starches and 

 sugars and those proteids which 

 have been changed into peptones, 

 are allowed to pass through it in 

 small amounts. During the later 

 period of digestion, when the stom- 

 ach becomes fatigued or consider- 

 ably emptied, the pylorus relaxes 

 more completely and permits 

 particles of considerable size to 

 pass through. The relaxation 

 under the normal conditions of health is sufficient to 

 empty the stomach at the end of from two and a half to 

 three hours. The stomach then enters upon a period of 

 rest, during which the glands are preparing for a subse- 

 quent period of work. 



1 Another ferment called rennin is found in the stomach, es- 

 pecially in babies and other sucking animals. Rennin has the 

 power of coagulating milk as a preliminary to its digestion. 



FIG. 71. Section through a 

 stomach gland, showing 

 its central canal and the 

 arrangement of its cells, 

 sh.) 



